The present invention relates to a multilayer ceramic electronic part incorporating therein a laminated structure composed of internal electrodes and ceramic layers, and external electrodes electrically connected to the internal electrodes; and, more particularly, to a multilayer ceramic electronic part which can be mounted on a circuit board and soldered thereon without causing cracks to be formed inside the laminated structure or the laminated structure to delaminate.
The multilayer electronic part can be, for example, a multilayer capacitor, a multilayer inductor, a multilayer piezoelectric part, a multilayer filter, a ceramic multilayer circuit board.
As the typical example of the multilayer electronic part, the multilayer ceramic capacitor includes a laminated structure having a plurality of ceramic layers made of a dielectric material and external electrodes, some of the ceramic layers including an internal electrode formed thereon. The ceramic layers are stacked on top of each other in such a way that the internal electrodes are alternatingly exposed at two opposing sides of the laminated structure. The external electrodes are disposed at the foregoing opposing sides of the laminated structure so as to be electrically connected to the internal electrodes of the laminated structure.
There is shown in FIG. 3 an arrangement of the foregoing laminated structure 3. The laminated structure 3 includes a plurality of ceramic layers 7, 7 . . . formed with internal electrodes 5, 6 and made of a dielectric material, and a plurality of ceramic layers 7, 7 . . . with no internal electrodes formed thereon. The ceramic layers 7, 7 with no internal electrodes formed thereon are known as dummy sheets. The ceramic layers 7, 7 are stacked in top of each other in such a way that the internal electrodes 5, 6 are alternatingly exposed at two opposing sides of the laminated structure 3. The dummy sheets are, respectively, disposed on top of an uppermost ceramic layer 7 and below an undermost ceramic layer 7. As shown in FIG. 1, at the foregoing opposing sides of the laminated structure 3 are, respectively, installed a pair of external electrodes 2, 2 so as to be electrically connected to the internal electrodes 5, 6.
Such a multilayer ceramic capacitor is not manufactured separately. To be more specific, first, a slurry is formed by blending together fine ceramic powders and an organic binder. The slurry is then thinly developed on a carrier film composed of polyethylene terephtalate film and dried thereon, thereby forming a ceramic green sheet. Next, after the ceramic green sheet is diced by a cutting head on a supporting film into a desired size, its one side is printed with a conductive paste and dried. As a result, as shown in FIG. 4, a plurality of ceramic green sheets 1a, 1b on which internal electrode patterns 2a, 2b are, respectively, formed lengthwise and crosswise are obtained.
Subsequently, after the ceramic green sheets 1a, 1b which are, respectively, formed with the internal electrode patterns 2a, 2b are stacked in top of each other, and ceramic green sheets 1, 1 which are not formed with internal electrode patterns are, respectively, stacked on top of an uppermost ceramic green sheet 1a and below a undermost ceramic green sheet 1b, they are pressed to form a laminated structure. When the ceramic green sheets 1a, 1b are stacked in top of each other, the internal electrode patterns 2a of the ceramic green sheets 1a are crossed with respect to the internal electrode patterns 2b of the ceramic green sheets 1b by a half of length of the internal electrode patterns 2b. Thereafter, the laminated structure is diced into a desired size to obtain laminated green chips. The laminated green chips are then sintered to obtain the laminated structure 3 as shown in FIG. 3.
Next, the conductive paste is applied on two opposing sides of the sintered laminated structure 3 and heated to form a pair of external electrodes 2, 2, resulting in the multilayer ceramic capacitor having a pair of external electrodes 2, 2 as shown in FIG. 1.
However, in such a laminated structure 3, an adhesion between the internal electrodes 5, 6 and the ceramic layers 7 is low compared to an adhesion between the ceramic layers 7. Particularly, at the foregoing two opposing sides of the laminated structure 3 to which the internal electrodes 5, 6 are exposed, the ceramic layers 7 get easily delaminated. Further, the fine cracks may develop inside the laminated structure 3.
Nowadays, in order to obtain a relatively large capacitance or a relatively large inductance using the laminated structure having a small size, a prevailing trend is to reduce the thickness of the internal electrodes 5, 6 or the ceramic layers 7. However, when the internal electrodes or the ceramic layers become too thin, a thermal stress generated at the internal electrodes or the ceramic layers may become too large during the soldering of external electrodes to land electrodes of the circuit, causing cracks to be formed inside the laminated structure or the laminated structure to delaminate.
It is, therefore, a primary object of the present invention to provide a multilayer ceramic electronic part having a high adhesion between ceramic layers constituting a laminated structure incorporated therein, and capable of preventing cracks from developing inside the laminated structure or the laminated structure from delaminating during the soldering of external electrodes on land electrodes of a circuit board.
In accordance with one aspect of the present invention, there is provided a multilayer ceramic electronic part incorporating therein a laminated structure having internal electrodes made of conductive particles and ceramic layers, the multilayer ceramic electronic part comprising first and second ceramic particles dispersed in the internal electrodes which are interposed between the ceramic layers, the first ceramic particles having an average particle diameter about or less than that of the conductive particle and second ceramic particles having such an average particle diameter that they extend both of the ceramic layers adjacent to each other with one of the internal electrodes disposed therebetween.
The ceramic layers are prevented from delaminated by the presence of the first ceramic particles and the second ceramic particles which function as thermal shock absorbers and anchors, respectively.
In accordance with another aspect of the present invention, there is provided a conductive paste for use in a multilayer ceramic electronic part, the conductive paste including conductive particles whose average diameter is about or greater than that of the first ceramic powders and is less than that of the second ceramic powders.